1. Field of the Invention
The present invention relates to a real image type viewfinder optical system in which an inverted image of a subject formed by an objective lens can be viewed as an erecting image.
Furthermore, the present invention relates to a real image type viewfinder optical system in which a viewfinder image can be observed both at eye level and at waist level.
2. Description of the Prior Art
Conventionally, in real image type viewfinder optical systems having an objective lens provided independently of a photo-taking lens, it has been so arranged that an inverted image of a subject is once formed by a positive objective lens such that an erecting viewfinder image is observed through an erecting optical system for inverting the inverted image of the subject vertically and horizontally. The known erecting optical systems can be roughly classified into two types, i.e. one employing a Porro prism or a Porro mirror as disclosed in U.S. Pat. No. 4,545,655 and the other in which an inverted image is re-formed as an erecting image by a relay lens system. The known real image type viewfinder optical system employing the relay lens has such a drawback that its relay optical system becomes excessively large in length and therefore, cannot be made compact in size. Meanwhile, as shown in FIG. 1, in the prior art viewfinder optical system employing a Porro prism 17 or the Porro mirror, a distance between an objective lens 1 and an eyepiece 6 becomes smaller than that of the type employing the relay lens but such a problem arises that a relatively large space is required to be provided vertically and horizontally for accommodating the Porro prism 17 or the Porro mirror according to shape of the Porro prism 17 or the Porro mirror.
Meanwhile, in the field of single-lens reflex cameras, a number of cameras have been proposed in which a viewfinder image can be observed both at eye level and at waist level. For example, as shown in FIG. 2, in a prior art viewfinder optical system disclosed in Japanese Patent Publication No. 31846/1973, a rooflike reflecting mirror 24 is employed in place of a pentagonal roof prism. The prior art viewfinder optical system further includes a plane reflecting mirror 21, a focusing screen 22, a condenser lens 23, an eyepiece 26 for eye level, a mirror 30 and an eyepiece 31 for waist level. At the time of eye level, the mirror 30 is retracted upwardly as shown by the solid lines so as to horizontally pass a bundle of rays through the eyepiece 26. On the other hand, at the time of waist level, a bundle of rays reflected from a roof face of the rooflike reflecting mirror 24 is vertically deflected by the mirror 30 disposed at the position shown by the imaginary lines so as to pass through the eyepiece 31. Therefore, the viewfinder image becomes an erecting image both at eye level and at waist level. At waist level, an enlarged image can be observed through the eyepiece 31.
However, in the above described arrangement of the prior art viewfinder optical system both for eye level and for waist level, since both the eyepiece 26 for eye level and the eyepiece 31 for waist level are disposed perpendicularly to the photo-taking lens (objective lens), a camera body incorporating this viewfinder optical system becomes large in height and thus, this viewfinder optical system cannot be used for a compact lens shutter camera.